Electrodes for use in aqueous alkali metal chloride electrolytes

ABSTRACT

THE INVENTION RELATS TO A PLATINUM METAL COATED TITANIUM ELECTRODE FOR USE IN ELECTROLYSIS OF BRINE. BY ELECTROPLATING THE PLATINUM GROUP METAL FROM AN ELECTROLYTE CONTAINING CERTAIN ORGANIC ADDITION AGENTS THE RESULTING PLATED ELECTRODE IS &#34;ACTIVATED.&#34;

United States Patent 3,592,750 ELECTRODES FOR USE IN AQUEOUS ALKALI METAL CHLORIDE ELECTROLYTES Denis Lee, Runcorn, England, assignor to Imperial Chemical Industries Limited, London, England No Drawing. Filed Oct. 21, 1968, Ser. No. 769,377 Claims priority, application Great Britain, Nov. 10, 1967, 51,218/ 67 Int. Cl. B01k 3/06; C01b 11/26; C23b 5/24 US. Cl. 204-95 11 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a platinum metal coated titanium electrode for use in electrolysis of brine. By electroplating the platinum group metal from an electrolyte containing certain organic addition agents the resulting plated electrode is activated.

The present invention relates to the manufacture of electrodes for use in aqueous alkali metal chloride electrolytes. More particularly it relates to the manufacture of coated titanium electrodes having an activated surface of a platinum metal with low chlorine-overvoltage characteristics.

In recent years it has been proposed to replace the conventional graphite anodes used in alkali metal chloride electrolytes, especially in cells electrolysing alkali metal chloride solutions for the manufacture of chlorine, hypochlorites and chlorates, by anodes comprising a core of titanium carrying a coating of a platinum metal, particularly platinum itself. In order to reduce the capital cost of these electrodes to an economically attractive level it is necessary to employ very thin layers of platinum, and these thin layers can most conveniently be produced by electrodeposition. Electrodeposition can easily be controlled to produce uniform thin coatings, the process operates at relatively low temperatures so that there is no danger of distorting the electrode structure by heating, and electrodeposited coatings have good resistance to wear in use, even as anodes in mercury-cathode cells where contact with mercury can damage platinum coatings. Although for these reasons the electrodeposition method is attractive, it has one serious disadvantage in that platinum deposits from conventional platinum-plating baths exhibit a high overvoltage for the liberation of chlorine when taken into service as anodes in alkali metal chloride electrolysis and the overvoltage quickly rises to even higher levels as electrolysis proceeds.

We have now found that this disadvantage can be overcome if there be added to the electroplating bath from which at least an outermost layer of the platinum metal is electrodeposited a compound selected from the group of additives defined hereinafter.

According to the present invention we provide a method for the manufacture of an electrode comprising a titani' um support and an electrodeposited coating of a platinum metal thereon, which is characterised in that at least an outermost layer of the platinum metal coating is electrodeposited from a plating bath which contains a compound of the class of additives which suppress maxima in polarographic waves associated with reactions carried out at a dropping mercury electrode, said compound being further selected from those which do not react with the platinum compounds in the plating bath.

Compounds which suppress maxima in polarographic waves as aforesaid are published for instance in The Principles and Applications of Polarography by G. W. C. Miller, Longmans, 1957. Certain of these compounds, for instance methyl red and methylene blue reduce the platinum metal compounds in plating baths and precipitate platinum metal powder. Such compounds are excluded from the scope of the invention by the definition in the preceding paragraph.

In the penultimate paragraph by a titanium support we mean a support consisting of titanium alone or an alloy based on titanium and having anodic polarisation properties comparable with those of titanium. Examples of suitable alloys are titanium-Zirconium alloys containing up to 14% of Zirconium, alloys of titanium with up to 5% of a platinum metal such as platinum, rhodium or iridium and alloys of titanium with niobium or tantalum containing up to 10% of the alloying constituent. By a platinum metal we mean one of the metals ruthenium, rhodium, palladium, osmium, iridium, platinum or an alloy of at least two of these metals.

In carrying out the method of the invention the amount of the said additive can vary over a wide range, and the operative range is not the same for all additives. Compounds of low solubility can be used up to the maximum solubility whereas compounds of high solubility are generally found to prevent electrodeposition of the platinum metal entirely if used at high concentration. The latter usually exert their best eifect at about half the minimum concentration which prevents electrodeposition. None of the compounds produces a useful eifect at a concentration less than 0.05 g./l. of the plating bath. The maximum usable concentration is readily determined by simple trial.

The preferred additive is gelatine, which may be employed at a concentration of 0.074.0 g./l. of the plating bath. We prefer to employ 0.5-2.0 g. of gelatine and particularly 1.0-2.0 g. of gelatine per litre of the bath. Other suitable additives are for instance agar-agar, gum arabic, high molecular weight polyethylene glycol, the sodium salts of long chain alkyl benzene sulphonates, peptone, glycerol and the sodium salts of alkyl aryl polyether sulphonates. Suitable amounts of these additives will be apparent from the examples shown hereinafter.

The whole of the platinum metal coating of the electrode may be deposited on the'titanium support from a plating bath containing an additive as defined hereinbefore. However, with the scope of this invention the titanium support may first be given a coating of a platinum metal by electrodeposition from a conventional plating bath and only a thin surface layer of platinum metal need then be deposited over the undercoating from the plating bath containing the aforesaid additive to produce an activated surface in accordance with the broad concept of the invention.

We have found that the electrodes produced according to the invention are particularly useful as anodes in aqueous alkali metal chloride electrolytes since the chlofine-overvoltage at the platinum metal surface is low at the start of electrolysis and does not increase appreciably with time. The electrodes may be employed, for example, as anodes in electrolytic cells wherein chlorine, hypochlorites or chlorates are produced by the electrolysis of alkali metal chloride solutions, as anodes in cells for the electro-dialysis of brackish water and as anodes for the cathodic protection of iron and steel structures immersed in sea water.

The invention is illustrated but not limited by the following comparative examples.

EXAMPLE l (Comparative example outside the scope of the invention) Platinum was electrodeposited on to a sheet of titanium which had been etched in 35% hydrochloric acid by making the etched titanium the cathode in a platinum plating bath containing hydrochloric acid at 1 molar concentration and 1 g./l. of chloroplatinic acid, using a current density of 0.3 a./dm. for minutes with the bath temperature at 70 C. The titanium sheet cathode was kept in motion in the bath with the plane of the sheet tangential to a circular path and a linear velocity of 160 cm./minute.

EXAMPLE 2 Platinum was electrodeposited on to etched titanium exactly as in Example 1 except that the plating bath also contained 1 g./l. of gclatine.

Each of the coated titanium specimens 1 and 2 was tested for development of chlorine-overvoltage by making it the anode in sodium chloride brine saturated with chlorine and containing 250 g./l. NaCl at a pH of approximately 2 and a temperature of 65 C. The potential of the anode was measured relative to a Luggin capillary probe positioned in the electrolyte close to the centre of the working anode face and connected by a salt bridge to a calomel reference electrode. Current was passed continuously between the anode and a cathode to give a current density of 4 ka./m. of anode surface and the anode potential was measured at the start, after 10 hours and, in the case of specimen 2, after 70 hours. The results reduced to overpotentials are shown in the following table.

Overpotential at 4 ka./m.

After 10 After 70 Example Initial, hours, hours, (specimen) mv. mv. mv.

EXAMPLES 3-9 4 overpotential at the anode, measured as in Example 2, was found to be 20 mv.

What we claim is:

1. A method for the manufacture of an electrode comprising a titanium support and an electrodeposited coating of a platinum metal thereon, which is characterised in that at least an outermost layer of the platinum metal coating is electrodeposited from a plating bath which contains a compound of the group consisting of gelatine, agar-agar, gum arabic, high molecular weight polyethylene glycol, the sodium salt of a long chain alkyl benzene sulphonate, peptone, glycerol and the sodium salt of an alkyl aryl polyether sulphonate.

2. A method according to claim 1, wherein before the outermost layer of the platinum metal is electrodeposited the titanium support is provided with a coating of a platinum metal by electrodeposition from a plating bath which is free from compounds of the said class of additives.

3. A method according to claim 1, wherein the amount of said compound is at least 0.05 grams per litre of the plating bath.

4. A method according to claim 1, wherein the said compound is gelatine.

5. A method according to claim 4, wherein the amount of gelatin employed is 0.07-4.0 grams per litre of the plating bath.

6. A method according to claim 4, wherein the amount of gelatin employed is 0.52.0 grams per litre of the plating bath.

7. A method according to claim 4, wherein the amount of gelatine employed is l.02.0 grams per litre of the plating bath.

8. A method according to claim 1 wherein the platinum metal of the coating is platinum.

9. An electrode comprising a titanium support and an electro-deposited coating of a platinum metal thereon prepared by the method of claim 1.

10. A process for the electrolysis of aqueous solutions of alkali metal chloride, characterised in that there is used as anode in said electrolysis an electrode according to claim 9.

11. A process according to claim 10 wherein the alkali metal chloride is sodium chloride.

Overpotential at 8 ka./m.

Concentra- Initial After 10 After 70 Example Additive tion g./l. mv. hours,mv. hours, mV

3 Agar-agar 20 2s 4 Gum arabic 2.5 10 5 Polyethylene glycol (molecular \vt. 15,00020,000) 10 26 6 Sodium dodecyl benzene sulphonate 30 80 7 Peptone 0.5 31 8 Glycerol 10 9 Sodium alkyl aryl poleythcr sulphonate L 0.5 20

1 Commercial product available under the trade name Triton X-200.

EXAMPLE 10 Platinum was electrodeposited on to a sheet of etched titanium exactly as in Example 1. A further coating of platinum was then applied over the initial coating under the same conditions of current density, time and temperature from an identical plating bath except that gelatine had been added to the bath in amount of 1 g./l. The coated titanium sheet was submitted to continuous longterm testing as an anode in sodium chloride brine containing 250 g./l. NaCl at a temperature of C. and pH approximately 2. The anodic current density was maintained at 8 ka./m. and after 4,100 hours operation the References Cited UNITED STATES PATENTS 3,055,811 9/1962 Ruff 20447X FOREIGN PATENTS 148,698 2/1937 Austria 20447 F. EDMUNDSON, Primary Examiner US. Cl. X.R. 

